1. Field of the Invention
The present invention relates to an electroluminescence (hereinafter simply referred to as “EL”) element and, in particular, to an upper electrode of an EL element.
2. Description of the Related Art
An EL element is self-emissive, bright, and has a wide angle of visibility. In addition, the thickness, size, and power consumption of a display panel which uses an EL element can be reduced to a same or greater degree compared to a liquid crystal display device. Because of these characteristics, wide application of EL elements is expected in the next generation of display device and light sources and, thus, much research and development is devoted to the EL element. Among EL elements, an organic EL element which uses an organic compound as a light emitting material particularly have attracted attention because a wide range of colors can be realized through design or selection of the organic compound, which facilitates realization of a full color display.
FIG. 1 schematically shows a cross sectional structure of a typical organic EL element. As shown in FIG. 1, an organic EL element has a light emitting element layer 30 having at least one organic layer including a light emitting material layered between a lower electrode 20 and an upper electrode 60, one of which functions as an anode and the other one of which functions as a cathode. The lower electrode 20 is a transparent electrode in which, for example, a conductive transparent metal oxide such as ITO (Indium Tin Oxide) is used. In the configuration illustrated in FIG. 1, the lower electrode 20 is formed below the light emitting element layer 30 and functions as an anode. The upper electrode 60, on the other hand, functions as a cathode in the configuration illustrated in FIG. 1 and is formed above the light emitting element layer 30 as a metal electrode in which Al or the like is used.
For the light emitting element layer 30, known structures include a single layer structure of an organic compound having both a charge (hole and electron) transporting function and a light emitting function or a multi-layer structure of two, three, or more layers including a charge transport layer and a light emitting layer. In the configuration illustrated in FIG. 1, a hole transport layer 304, an emissive layer 306, and an electron transport layer 308, each of which is formed using an organic compound, and an electron injection layer 310 made of LiF or the like, provided for improving electron injection efficiency from the upper electrode 60 which functions the cathode to the electron transport layer 308, are layered, in that order, from the side of the lower electrode 20 which functions as the anode.
In an organic EL element having such a structure, holes are injected from the anode side and electrons are injected from the cathode side, the organic light emitting material in the emissive layer 306 is excited with an energy of recombination of the injected holes and electrodes, and light is emitted when the organic light emitting material returns to its ground state.
In the organic EL element of the related art as described above, when a low-molecular weight organic compound is used as an organic material which is a part of the light emitting element layer 30, each layer of the light emitting element layer 30 is formed through a vacuum evaporation method. Regarding the upper electrode 60 (which is the cathode in the illustrated configuration) to be formed over the light emitting element layer 30, a similar vacuum evaporation method is used to form the upper electrode 60 after the light emitting element layer 30 is formed.
It is known that the organic compound contained in the light emitting element layer 30 is easily degraded by exposure to moisture, oxygen, and other impurities. By consecutively forming the upper electrode 60 through evaporation after the light emitting element layer 30 is formed, it is possible to form the upper electrode 60 without breaking the vacuum environment during the formation of the light emitting element layer 30, that is, without exposing the upper surface of the light emitting element layer 30 to the external atmosphere, to thereby prevent degradation in characteristics.
There has been a problem, however, in that the film formation rate in vacuum evaporation is low, and, in particular, because the upper electrode 60 is formed to a thickness which is larger than that for the other layers, the productivity in formation of the cathode through vacuum evaporation is low.
Moreover, in general, a film formed through evaporation has low uniformity and low coverage on the formation surface. Therefore, when the upper electrode 60 which is at the uppermost layer of the organic EL element and having a large unevenness on the surface due to various steps in the lower layers is formed through evaporation, problems tend to occur in which coverage deficiency occurs at the portion corresponding to steps, to cause disconnection.
A film formed through sputtering, on the other hand, has high coverage and the sputtering achieves a high film formation rate. Therefore, the problems noted above can be resolved by forming the upper electrode 60 through sputtering. However, during formation of the upper electrode 60 through sputtering, high energy electron beams, high energy neutral atoms, high energy ionized atoms, ultraviolet rays, etc. are generated and the light emitting element layer 30 already formed below the upper electrode 60 and made of an organic material may be easily damaged. When the light emitting element layer 30 is damaged, degradation in the light emission characteristics of the organic EL element may occur such as, for example, reduction of light emission efficiency and generation of a “dark spot” which is a point of light emission deficiency.
Japanese Patent Laid-Open Publication No. Hei 11-162652 (hereinafter referred to as “Reference 1”) discloses protection of the lower organic layers and improvements in the productivity by employing a two-layered structure for the upper electrode including a protection electrode layer formed through vacuum evaporation and a main electrode layer formed through sputtering, in which the main electrode layer is formed through sputtering after the protection electrode layer is formed through evaporation.
By employing, as the upper electrode to be formed above the organic layer, a two-layered structure including a layer formed through evaporation and a layer formed through sputtering as disclosed in Reference 1, it is possible to protect the organic layer to a higher degree than when a configuration in which the upper electrode 60 is directly formed above the organic layer through sputtering is employed, and to improve the productivity at the same time.
The applicants found, however, that, in reality, the light emission characteristic of the organic EL element employing such a two-layered structure can be improved only by a small degree compared to the configuration in which the upper electrode is formed directly above the organic layer through sputtering, and there are some cases in which the light emission characteristic is reduced, and therefore, the expected advantages cannot be obtained. A reason for this may be that the damages to the light emitting element layer below the protection electrode layer formed through evaporation is not sufficiently prevented.